The present invention relates to an apparatus for the fixation of a single-tooth restoration to an implant in the form of a fixture which is permanently anchored in the maxillary.
It is previously known in the art to permanently anchor oral and extraoral prostheses in bone tissue by means of screw-like anchorage elements, so-called fixtures, made of titanium. The method which was proved to give the highest anchorage stability and which has successfully been used clinically for more than 20 years is the so-called osseointegration method developed by professor PerIngvar Br.ang.nemark et al and described, for example, in:
Br.ang.nemark/Zarb/Albrektsson: "Tissue-Integrated Prostheses", Quintessence Books, 1985.
The method is based on a highly exact and atraumatic implant technique of the fixture such that a direct contact, in other words an exact adaptation without interjacent soft tissue, occurs between the fixture and the bone tissue. Such a direct contact between fixture and bone tissue provides the best preconditions for a really permanent fixation of, for example, a dental prosthesis.
The helicoid fixtures of pure titanium are operated into the maxillary in a first surgical operation which is followed by an unloaded healing phase of critical length during which the fixture is covered by intact mucous membrane. During this healing phase, the bone tissue grows into and forms a unit with the implanted fixture. In a second operation, the fixture is then exposed and a substantially tubular spacer is applied to the fixture by means of a spacer screw. Subsequently, the dental prosthesis for example--in the form of a bridge construction, is united to the fixture by means of a fixing screw which, in its turn, anchors in the spacer screw.
The development work which has hitherto been carried on in this art has, to a great extent, been concentrated on the surgical technique, see for example the above-mentioned publication "Tissue-Integrated Prostheses", and on the design of the fixture. Apart from a special surface structure and surface-chemical composition, it is, thus, important that the fixture be given an adequate, geometric design in order to more readily to satisfy the requirements of both atraumatic surgery and surface cleanliness. Swedish printed application No. 332 486 discloses a fixture in the form of a screw which is exteriorally threaded. The threaded portion of the screw is inserted in a threaded bore in the maxillary on permanent implantation of a dental prosthesis.
Swedish patent application No. 86.00611-1 also discloses a self-threading fixture which makes for a slightly simplified operation technique, but nevertheless satisfies all the requirements for osseointegration to be achieved. The fixture is designed as a cutting, self-threading tool which, in a single operational phase, is screwed into the pre-drilled seat. In this instance, a minor number of instruments are required for preparing the bone tissue, and in particular the prior art double-tap preparatory phases are eliminated.
While the osseointegration method provides uniquely advantageous properties with respect to anchorage stability and the risk of loosening or detachment, complications may, in certain cases, set in which entrain detachment tendencies. For example, extreme oral loadings, occlusion and chewing may entail the risk of detachment. Furthermore, incorrect surgical implantation of fixture and spacer may jeopardize the osseointegration. Consequently, to reduce the mechanical stresses involved, the dental prostheses are, as a rule, anchored by means of a bridge construction with the assistance of a plurality of fixtures, for example six (see further the publication "Tissue-Integrated Prostheses"). Should any of the fixtures display a tendency to loosen, the remainder will then ensure that the reliable overall anchorage is still maintained.
However, in recent years attempts have been made within this art to offer reliable anchorage of individual teeth, so-called single tooth restoration, see, for example, "Modified single and short-span restorations supported by osseointegrated fixtures in the partially edentulous jaw", by T. Jemt, The Journal of Prosthetic Denistry, Feb. 86, Vol. 55, No. 2. In such single-tooth restoration, one fixture alone should be able to withstand all of the oral loadings, such as the torque, flexion and compressive forces which may occur. The torque loading is of particular importance, as it strives to loosen the screw connection between the spacer screw and the fixture.
Naturally, for single-tooth restorations and the extreme oral stresses to which these may be exposed, it is of vital importance that the operation technique and the design of the fixture be of the optimum degree. However, it is also important that the design and anchorage of the spacer, or "abutment" as it is often called in this art, be of the best conceivable standard in order to prevent loosening of the screw connection.